This invention relates to a head exchange system for a disc drive system for controlling the position of a head so as to cancel, during head exchange, a previously measured offset value between the head and a track of a recording disc.
In the exchange of the head due to read access or write access of the disc drive system, the offset value is different for each head. Therefore, the offset value is previously measured for each head using a data-surface-servo system, and measurement for off track is taken to control the position of the head in order to cancel, during head exchange, the offset value between the head and the track of the recording disc.
Examples of prior art offset correction systems are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-222465 and U.S. Pat. No. 4,136,365. In such prior art offset correction systems, the offset value of the head is first measured using a data-surface-servo system as described therein.
Measurement of the offset value is automatically carried out in a certain time interval. A memory device, such as a RAM of a MPU, of the disc controller maintains an offset value of each head measured until the next automatic adjustment.
Namely, the method steps are as follows: moving the head to the servo track of the data surface by a head positioning servo circuit, then reading servo data from the servo track by the head, then measuring the offset value from the servo data by controlling a microprogram of the MPU, and then storing the offset value of each head to the memory device, such as the RAM of the MPU, of the disc controller by controlling the microprogram of the MPU.
In an ordinary write or read operation, the offset value is applied from the memory of the MPU to a circuit receiving a position signal from a position encoder of a head access motor of a disc drive mechanism so that the measured offset value of the selected head is cancelled. The circuit for the position signal having completed the offset correction causes a control current to flow through a motor coil of the head access motor of the disc drive mechanism through a power amplifier to move the head, for which the offset value has been cancelled, to the on-track position.
Since physical or actual time is necessary to realize head movement for cancelling the offset value, a busy signal is transmitted to the host controller for a constant time in accordance with the head moving time for the expected maximum offset value, the read or write access is set to the waiting condition until the end of the head movement, and thereafter the read or write operation is started by resetting the busy signal upon completion of the head movement.
In the prior art head exchange systems using such offset value cancellation, the busy signal must be transmitted to the host controller to set it to the waiting condition until the end of the head movement or offset value cancellation. Such a waiting time creates a problem in that the processing ability of the disc drive system is lowered and high performance of the system is adversely affected.